Lithium in the form of lithium carbonate is administered to manic-depressive patients. The therapeutic range of lithium ion in plasma is quite narrow, namely, 0.8 to 1.2 mM. It is important to monitor the lithium level in such patients because of the toxic side effects that appear when the lithium level in blood exceeds the recommended level.
It is known to assay lithium quantitatively using solution assays and ion-selective electrodes. Dyes, such as 14-Crown-4-ether derivatives are known for use in such assays. Japanese Kokai 62/72683 (1985) discloses a class of such dyes that act as colorometric reagents in extracting lithium from solutions and as charge transport carriers for ion-selective electrodes. The problem is that none of the known 14-crown-4-ether derivatives can be adapted for use in dry analytical elements. Moreover the above "wet" methods involving solution extractions or ion-selective electrodes suffer from severe ion and other interferant problems. It would be desirable to have a dry analytical element for assaying lithium that obviated these problems.